Microwave techniques are known for measuring certain physical parameters within a gas turbine engine, such as blade tip clearance, which is the distance between the end of the blade and the turbine casing. Gas turbine engines provide some of the hottest environments for temperature measurements. Gas path temperatures can exceed 2000° F., which is beyond the melting points of most metals.
Antennas are typically used to transmit and receive electromagnetic energy within ambient temperature environments and in connection with a variety of devices, such as mobile phones, radios, global positioning receivers, and radar systems. Microwave sensors can also include one or more antennas to support the propagation of microwave signals within the desired measurement environment of a gas turbine engine. As an electromagnetic wave, the microwave signal will typically diverge and cover a larger area as the energy propagates further away from the antenna. The beamwidth characteristic of an antenna is typically measured as the angle at which the transmitted power is 3 dB below the on axis, or boresight power. Antennas used with microwave sensors for gas turbine engine measurements can have beamwidths that are 90 degrees or larger.
Turbine engines typically have various internal stages comprising a set of blades attached to a rotating disk. A microwave sensor can be mounted through a hole or attached to the inside of the engine case to enable the antenna to cast its beam onto the blades, which will be rotating by the antenna during engine operation. The blades rotate close to the position of the sensor within the casing, typically in the range of 0.1 to 1 inch separation between each blade and the sensor position. For accurate measurements, such as blade tip clearance or time of arrival, it is desirable to receive energy only from the tip area of the blade. Microwave signals travel much further distances than 1 inch, however, resulting in the transmission of microwave signals that travel past the blade tips to the edges of the blade or other parts of the engine. Therefore, the resulting signal received by the microwave sensor often contains reflections from other objects that are not the target of interest, commonly referred to as clutter.
Conventional techniques for removing clutter include (i) range gating or (ii) modeling clutter statistics and applying subtraction techniques to remove the influence of the interfering signal. Often, the clutter is close to the target of interest, a few inches away or less. For example, for the typical turbine engine measurement scenario, the bandwidth for range gating would be several GHz or more, which is impractical due to cost and difficulties in antenna design. Clutter subtraction techniques are insufficient for typical turbine engine measurements because the measurement of turbine blades using a microwave sensor require phase accuracies of less than one degree. Current clutter removal techniques are not capable of addressing this phase accuracy requirement. Therefore, another method of removing clutter from microwave sensor measurements is desirable for the operating environment of a gas turbine engine.
The output waveforms for a typical microwave sensor used for blade measurements of a gas turbine engine have complex features as a result of microwave signal interaction with a complex turbine blade geometry. The identification of a single point on an output waveform for tip clearance or time of arrival measurements can be difficult as a result of thee complex signal features. Typical peak detection methods, such as a polynomial curve fit, are too computationally intensive for the measurement of blades in real-time. Methods for finding a single point on the blade with the highest return signal can present variable results as the blade twists and changes dimensionally during normal operation. Nevertheless, a well defined, repeatable point for the engine blade is desirable for most blade tip measurements applications.
In view of the foregoing, there is a need in the art for adapting the use of a microwave sensor to achieve blade measurements that are based on a repeatable point for the blade tip by minimizing the influence of interfering clutter signals.